Method of minimizing boron oxide deposits in a jet-type engine



July 10,1962

INVENTORS IN A JET-TYPE ENGINE J. MACATICIAN ETAL METHOD OF MINIMIZING BORON OXIDE DEPOSITS Filed Dec.

FWJE as; .PuJE E 021600 hydride, 'decaborane, organoboranes, including the lowel alkyl 3,043,100 METHQD F MINIMEZING BORUN OXIDE DEPOSETS 1N A .iET-TYPE ENGENE John Macatician, Lake Hopatcong, NJ and Stephen S. i-iubard, deceased, late of Lake Hiawatha, N.J., by James W. Lewis, administrator, Ealtimore, MIL, assignors to Thiokol (Jhemical Corporation, Trenton, N 1., a corporation of Delaware Filed Dec. 11, 1958, Ser. No. -779,794

1 (Ilaim. ((11. 60-354) 7 This invention relates to a method of minimizing boron oxide deposits on surfaces within jet-type aircraft engines operating on a boron containing fuel. By boron containing fuel is meant a high-energy fuel such as a boron including diborane, the pentaboranes, and

pentaboranes and lower alkyl decaboranes, and conventional hydrocarbon jet fuels containing the same. Lower alkyl pentaboranes can be prepared, for example, according to the methoddescrihed in application Serial No. 546,803, filed November 14, l955,'now abandoned, of Jack R. Gould and John E. Paustian. Lower alkyl decarboranes can be prepared, for example, according to the method described in application Serial No. 557,634, filed January 6, 1956, now U.S. Patent No. 2,987,552, to Joseph A. Netf and Edward]. Wandel.

The present invention can be applied to any of the three basic types of jet engines, i.e. the ram-jet, the turbojet, and the turbo-prop, although it has particular application to the turbo-jet and turbo-prop type engines, and will be further describedas applied to a turbo-jet engine.

In the operation of a turbo-jet aircraft engine, air flows into the engine through the air entrance section and then into the compressor section, where it is usually compressed to a pressure of about 45 to 180 p.s.i.g. The air entrance and compressorsections may follow any one of several designs; and the compressor section may possess either an axial or a centrifugal compressor. If of the centrifugal type, the compressor may additionally possess either a singleor a double entry.

From the compressor section, the compressed air flows into the combustion section where it is combined with a metered and atomized or prevaporized amount of fuel and its temperature increased by combustion of the fuel. It will be noted that the air flow in this section is such that only a relatively small amount of the air actually mixes with the fuel at the point of combustion. This portion of the air is generally referred to as the primary air supply. The weight ratio of primary air to fuel is generally between 10 to l and 50 to l.

Following the combustion of the fuel, the combustion products are almost immediately and intimately mixed with the remaining or secondary air. Thus, the combustion products are cooled from a combustion temperature of about 3500 to 4000 F. to an average temperature of about 1400 F. The latter temperature is dictated at the present time by the types of metals and metal alloys that are presently available for use within the turbine section of an engine.

The gaseous products of combustion and the excess air entering the turbine section from the combustion section cause the turbine rotor or rotors to revolve and to drive thecompres sor in the compressor section and also auxiliary equipment such as fuel pumps, lube oil pumps, generators, etc.

The gases leaving the turbine then flow into the tailpipe section from whence they vent to the atmosphere. The design of this section may vary considerably. For example, it may have either a single or double exit,

and it may also be ofthe variable orifice or adjustable.

3,43,lfl Patented July 10, 1962 burning combustion conditions. The tailpipe section normally operates at temperatures of about 900 to 1400" F.

When afterburner nozzles are provided in the tailpipe section, a difluser is usually placed between the turbine section and the nozzles. This device serves to redistribute the gas flow in the tailpipe and topromote better combustion of the fuel issuing from the afterburner nozzles.

The combustion section may be any one of the conventional types as, for example, one that employs multiple combustion chambers (cans) or one that uses an annular combustion liner or chamber (a burner basket).

In the first of these types, the air flow is split upon leavingthe compressor and equal portions sent to each can,

Where these portions are combusted with portions of the fuel. The combustion products are then recombined with.

secondary air and routed to the turbine section..

When the combustion section is of the burner basket type, the primary portion of the air is diverted from the main stream and directed toward the fuel injector within the basket where it burns with the fuel. The remaining or secondary air is then mixed with the products of combustion at a point prior to their entrance into the turbine section.

The turbine section of a jet engine may contain one or more turbine rotors and one or more stages. In addition,

the turbine blades may be of the impulse and/ or reaction types and may or may not be' shrouded. Associated with the turbine rotor blades are stator blades which direct the hot gases against the rotor blades.

One of the more serious problems associated with the use of boron containing fuels in turbo-jet aircraft engines results from the formation of boron oxide as a combustion product and its subsequent deposition on the surfaces of the combustion section, the turbine section, and the tailpipe section. For example, a pound of diborane produces 2.5 pounds of B 0 and a pound of pentaborane, 2.76 pounds. The glass-like boron oxide has an approximate melting point of 1070 F. and has a high viscosity at turbine operating temperatures of about 1400 to 1600- eluding afterburner parts, and variable-area nozzles.

' It has been proposed to minimize depositsof boron oxide in turbo-jet engines by avoiding the strong reverse flow that is usually designed into a high velocity combustor to provide flame piloting. The wide flammability limits and high flame speeds of boron fuels make it possible to reduce the piloting otherwise required'by hydrocarbon fuels. It has also been proposed to minimize boron oxide deposits by filming the combustor Walls with air to prevent impingement of boron oxide on the surfaces" thereof. Various methods of providing the air film are available such as porous walls, louvers, step construction, etc. These proposals, however, have not been successful in alleviating the boron oxide deposition problem.

In accordance with the present invention, boron oxide deposits are minimized within the combustion, turbine and exhaust sections of an air-breathing jet-type aircraft engine operating on a boron containing fuel by introducing within the appropriate section a material selected from the group consisting of hydrogen fluoride and boron trifluoride in amount suflicient to completely remove the boron oxide deposits, where transpiration occurs.

The hydrogen fluoride or boron trifluoride can be introduced within those sections where boron oxide is prone to form by any one or more of several known means including introduction within the air or combustion products air stream flowing to the pertinent sections and introduction directly into the pertinent section. The fluoride can be introduced within the combustion section, for example, by incorporation into the primary or secondary air stream from the compressor section. The fluoride can be introduced Within the turbine section, for example, by direct injection as by utilizing a porous material, such as a ceramic or sintered stainless steel, for construction of the turbine stator blades and transpiring the fluoride through the stator blades. The fluoride can be introduced within the exhaust section by direct injection into the gas stream flowing therein. Where an afterburner is employed, the fluoride can be injected at or subsequent to the diifuser and also in the air stream supplied to the after-burner.

In order to test the efficiency of hydrogen fluoride and boron trifluoride in minimizing the deposit of boric oxide on heated surfaces, a combustion and exhaust system simulating that of a jet-type engine Was constructed and is EXAMPLE 11 In this example boron trifluoride was transpired through the probe. The following results were observed.

Boron trifluoride transpired- 2.856 grams per second. Weight of deposit on the probe 0.1770 gram or a negligible amount.

shown in the attached drawing. In the drawing, numeral 1 represents a section of 3 inch steel pipe about 55 inches The fOllOWiIlg table Summarizes eXamples Showing the l d di id d i t Sections b flange 2, Thi effect of operation in the absence of a fluoride and opi comprised h t l h ll of h sy tem d di eration in which a boron oxide deposit was permitted to posed within it were two sections, 3 and 4, of 1,81 i h build up prior to transpiring a fluoride through the depoinside diameter type 321 stainless steel tubing joined endsition probe. In Examples IV a d the fluoride W4s to-end at flange 2. Section 3 was 16 inches long, was transpired through the probe just prior to shut-down.

Table Total Tran- Fuel Air Tran- Tran- Running spirant Total Probe Gas Rate, Rate, spirant Example sprirant Time Running Deposit Temp. Temp. (grams/ (grams/ (grams/ (min) Time (Grams) 0.) 0.) see.) sec.) sec.)

(min) III None--. 8 None. .796 975 1,000 3.0 32.1 0 IV 8V .327 950 1,000 3.0 32.4 .152 (for v BF=- 8% .275 950 1,000 2.7 28.9 z tgor 30 evenly perforated and defined the combustion chamber In Example III, the probe was completely covered with liner. Section 4 was 36 inches long and defined the exglass-like 13 0 with large glass 'beads on the bottom and haust chamber liner. Air entered the combustion chamtrailing side. In Examples IV and V, the probe was covber, through inlet 5 and the perforations in liner 3. Fuel ered in part by a gray ash. entered the combustion chamber through inlet 6 and In addition to hydrogen fluoride and boron trifluoride, spray nozzle 7. Spray nozzle 7 was a standard 80 holunsymmetrical difluoroethane (CI-IF --CH boiling point low cone oil burner nozzle with a 1 to 2 gallon per hour -25 C.) can be used to minimize boron oxide deposits rating. Spark plug 8 just down-stream of the fuel nozzle was used for ignition. Cooling air Was injected tangentially between the shell 1 and exhaust chamber liner 4 through air inlet 9. Thirty inches down-stream of flange 2, deposition probe 10 was inserted into the ex haust chamber. Probe 10 was a short length of tubing (about 0.5 inch in diameter and 1.68" long) closed at one end and fabricated from a sintered stainless steel having a mean porosity of 10 to 12 microns. Fluid to be transpired was forced into probe 10 through fluid inlet 11. The temperature of probe 10 was measured by a thermocouple attached thereto and the exhaust gas temperature was measured by means of a chromel-alumel thermocouple located 1 inch up-stream of probe 10. Both the probe and exhaust temperatures were continuously recorded. This combustion and exhaust system was employed in each of the following examples. In each of the examples, the fuel supplied to fuel inlet 6 was a 70 30 percent by weight azeotrope of trimethylborate and methanol.

EXAMPLE I In this example, hydrogen fluoride was admitted to inlet 11 prior to initiating combustion of the fuel. Burning continued for 240 seconds, producing an exhaust gas temperature of 1050 C. During the run the trimethylbora-te containing fuel was consumed at the rate of 3.717 grams per second and air was admitted to the combustion chamber at the rate of 29.147 grams per second. Hydrogen fluoride was transpired through the probe at the rate of 0.024 gram per second and the exhaust velocity of gases passed the probe at 234 feet per second. The weight of the clean probe previous to the test was 242.9230 grams and after 243.0187 grams. The weight of the deposited material was 0.0951 gram or practically a negligible amount.

according to the method of this invention.

The amount of fluoride required to be introduced Within the various sections of the engine in order to minimize the deposition of boric oxide varies greatly with the characteristics of the engine and the conditions under which it operates. For example, in ram-jets, where less of a deposit problem exists than in turbojets, less fluoride is required. In general, however, the total amount of fluoride introduced within the various sections is sufficient to completely remove the boron oxide deposits, where transpiration occurs.

We claim:

A method of minimizing boron oxide deposits within the combustion, turbine and exhaust sections of an airbreathing jet-type aircraft engine operating on a fuel containing at least one material composed only of carbon, oxygen, hydrogen and boron atoms selected from the class consisting of boranes and organoboranes which consists of introducing within such a section in contact with surfaces upon which boron oxide would normally deposit a material selected from the group consisting of hydrogen fluoride and boron trifluoride.

References Cited in the file of this patent UNITED STATES PATENTS 1,666,693 Gaus Apr. 17, 1928 2,744,380 McMillan et al. May 8, 1956 2,784,160 Blaker Mar. 5, 1957 OTHER REFERENCES Booth et al.: Boron Trifluoride and Its Derivatives, John Wiley & Sons, Inc., New York (1949), p. 33.

Sowa et al.: Journal American Chemical Society, vol. 57, Jan-June 1935, pp. 454-456.

Hughes et 211.: Industrial and Engineering Chemistry. vol. 48, No. 10, October 1956, pp. 1858-1862. 

